One mistake many of us make is equating the slavery issue, as the cause of the war, to the reason the individual served his country, when in fact the reasons could be totally divergent.
I don't recollect anyone declaring that Johnny fought for slavery or that Billy fought to free the slaves. According to -- close your eyes, Joanie -- McPherson's For Cause and Comrades, and -- you can open them now -- Chandra Manning, more than a few did.

When western Billy, who had likely never seen a black (free or slave) got into slave territory, he soon realized that the peculiar institution had to go: winning the war and leaving slavery intact would lead only to another war. He evinced this idea many months before Lincoln acted on that feeling.

Johnny, of course, didn't own a single slave, so he couldn't have been fighting for slavery. Channing and McPherson (don't tell Joanie) take it a bit further. Possibly most fought for the adventure, the pay, an obligation to friends and family -- maybe even a few to protect their home; but a good many did join to keep alive the idea that one day they would be wealthy enough to own a slave, or simply because they feared having free blacks loose among them. -- the old and more modern argument that the black man wants nothing so much as he wants to marry your wives and daughters.

But Johnny knew, even before Billy did, that the war was about preserving slavery. He might not have liked that, but now he was in and couldn't very well walk out.